chore: release 1.13.1 (#482)

diff --git a/setup.py b/setup.py
index 0930fd8..cb6ed5a 100644
--- a/setup.py
+++ b/setup.py
@@ -30,7 +30,7 @@
 with io.open("README.rst", "r") as fh:
     long_description = fh.read()
 
-version = "1.13.0"
+version = "1.13.1"
 
 setup(
     name="google-auth",